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Basic Information

The habitability of a planet or other body is either the ability for it to sustain human life, or to sustain any life at all. The former, of course, is much narrower. We need water to drink, mild temperatures, and comfortable pressures. The high oxygen levels we need to breathe come from photosynthesis, worlds that are habitable to humans are likely to have photosynthetic life already, though different quirks in the biochemistry such as Alien Amino Acids mean we may not get much nutrition from eating the local flora and fauna. Habitability runs a spectrum between environments that would kill within instants, and locations that could support unaided human settlements.

A planet that is the right distance from the star to support liquid water on the surface is known as a Goldilocks Planet. Any world that has liquid water is habitable to bacteria at least, but humans may not be comfortable: water is liquid for a full hundred degrees, and pressures may be quite low or high. We might be limited to the polar regions or the highest mountains on even apparently habitable planets. In the same way, a planet may have underground water or a cold water-ammonia mixture that is useless to humans, but terrestrial bacteria or any native life might thrive on. Antarctica, meanwhile, is a location on a habitable planet that only the hardiest native life survives, and for humans is uninhabitable in the long-term.

What about alien species? They would be adapted to survive the conditions of their home planet, but a spectrum of habitability would still exist for them. When it comes to Hypothetical Types of Biochemistry, the solvent is a key indicator to the temperature ranges they need: a solvent that is liquid at colder temperatures means that they will prefer colder planets. Note that habitable ranges are overlapping. Antarctica may be slightly habitable to a species that uses Ammonia as Biological Solvent, as the winters are cold enough for ammonia to be liquid, but the toxic oxygen and lack of ammonia to drink would deter them.

It isn't necessarily a mutual matter. Chlorine-Breathing Life might just need a respirator during extended activity to survive our planet, while we'd die quickly if exposed to their planet due to chlorine toxicity and the lack of an ozone layer. The underlying building blocks are not as important as the requirements of temperature and respiration that come out of them, either. Silicon-Based Life might breathe our atmosphere and drink our water without difficulty, needing only to grow their plants on our silicon-rich rocks for nutrition. If it turns out that life made from silicon tends to use ammonia as a solvent, they would probably need to live on worlds that can sustain liquid ammonia.

Many worlds should be able to support sealed habitats that humans can live safely within. A cold surface can have heated chambers built on it. A human could walk around in a vacuum suit or a diving suit, depending on the conditions, with internal heating elements. A long-term settlement might be extensive and self-sustaining with plenty of room in underground chambers, surface domes, Hydroponics to supply food, mechanical resource extraction, and so on. Trying to cool a habitat on a hotter planet is just not going to work, however, so colder environments are better for sealed habitats.

An uninhabitable planet may be made habitable through Terraforming, depending on the circumstances. A lack of water can be addressed through icy comet bombardment, low temperatures with greenhouse gases, high temperatures with orbital sunshades, and lack of oxygen with green bacteria and plants. As soon as habitability is addressed, a food chain can be established by adding new plants and animals. When applied to an alien species with different preferences trying to make a planet habitable to them, this is known as Xenoforming.



Game and Story Use

  • In a sci-fi or fantasy setting with multiple species, humans might be limited to a few extreme habitats on a planet while others roam widely, or vice versa.
  • A cosmopolitan sci-fi city where many species live together is a possibility, and there's plenty of room for friction arising from the non-fatal differences between them.
    • Even more so when designing space stations - accomodation for some species might be a serious challenge.
  • When life spreads from one world to another, it might act as an Invasive Species if it is better suited to an environment than the native ecosystem, especially a marginal habitat.
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